ALUMINA. (FR., Alumine ; GER., Alaunerde.) Formula the only oxide of aluminium known, is a very large constituent of the earth's crust. In combination with silica, it enters into the composition of all slatey and clayey earths, and of many rocks, minerals, and shales. It is seldom found in the pure state, except in varieties of the mineral corundum, such as ruby, sapphire, &c. Emery is a less pure variety of the same mineral, which, on account of its extreme hardness, has received numerous industrial applications. As the hydrate, alumina occurs in diaspore, hydrargyllite, gibbsite, and other minerals. In bauxite, so called from Baux in France, whence it is obtained, alumina exists in proportions varying from 60 to 75 per cent. of the wholo substance. The following is the average composition of bauxite:— Alumina Magnesia .. 0.38 Silica .. 6.29 Soda .. Oxide of Iron .. .. 2.40 Potash .. 0.46 Lime .. 0.55 Water .. Alumina is insoluble in water, but soluble in acids and alkalies; with the former it combines to form the ordinary salts of aluminium, but with the latter it plays the part of an acid, forming the salts called " aluminates." Crystalline alumina has a specific gravity of 3.9, and, next to the diamond, it is the hardest substance known. Wben pure, it is infusible in all temperatures except that of the oxy-hydrogen flame.
The strong affinity exhibited by alumina for vegetable colouring matter renders it invaluable to the dyer and calico-printer, and upon this property, in conjunction with its powerful attraction for all vegetable fibres, depends its extensive use as a mordant (see Dyeing). By combining, first, with the colouring matter, and then with the fibres of the substance to be dyed, the cloth and the colouring matter are brought into very intimate union with each other. In the form of clays of various kinds, alumina is largely employed in the manufacture of pottery of all descriptions (see Clay). The uses of alumina in the form of emery, alluded to above, are well known.
There are various methods of preparing alumina in a pure state. It may be obtained from common potash alum by heating it with a solution of carbonate of ammonia ; the hydrate of alumina precipitated is well washed, dried, and ignited, the residue consisting of pure alumina. A better
method is that of igniting ammonia alum, by which its volatile constituents are driven off, and alumina ia a tolerably pure state is left behind.
Clay, or minerals containing alumina, digested with a concentrated solution of potash or soda, yields an alkaline aluminate, from which hydrated alumina may be precipitated by passing a stream of carbonic acid through the solutions, or by heating it with chloride .of ammonium or acid carbo nate of soda; the hydrate is dried and ignited sa in the previous case.
Alumina in a state suitable for the preparation of the pigments known as " lakes" may be produced in the following manner : " Dissolve I lb. of alum in gallon of water, and add 75 grains of aulphate of copper, and about lb. of zinc turnings; leave the mixture for three day-a in a warm place, renewing the water lost by evaporation. The copper is first deposited upon the zinc, the two metals thus forming a voltaic couple sufficiently atrong. Hydrogen is disengaged, sulphate of zinc is formed, and the alumina gradually separates in the state of a very fine powder ; the action is allowed to continue until there is no more alumina left in solution, or until ammonia ceases to give a precipitate. If the reaction is prolonged beyond this point, oxide of iron will precipitate if present. The alumina washes easily, and does not contract upon drying."—(Derniers Progres de l'Indastrie Chimique.) Sulphate of salt is obtained by dissolving alumina iu sulphuric acid. It is now largely employed as a substitute for alurn under the name of " cone,entrated alum." When clay is used for the preparation of sulphate of alumina, the iris) is removed by adding, forroeyanide of potash to the dilute solution of the sulphate, whereby Prussian blue is precipitated. Sulphate of alumina, owing to its variable composition, may not be always safely used in dyeing and calico-printing instead of alum, but in the majority of eases it is a thoroughly effective substitute (see Alum).